This invention relates to an apparatus for measuring intermodulation distortion and particularly to an intermodulation analyzer which automatically switches its output depending upon the type of intermodulation distortion test being conducted.
Intermodulation may be defined as: The production in a nonlinear circuit element of frequencies corresponding to the sum and differences of the fundamentals and harmonics of two or more frequencies which are transmitted through the element. Thus, when two frequencies are applied to the input of an audio frequency device, not only do the fundamental frequencies appear in the output but also sum and difference frequencies which are, as a rule, not harmonically related to the fundamental frequencies.
Several techniques and various instruments have been developed to measure intermodulation distortion. Two of the more widely used tests are the SMPTE (Society of Motion Picture and Television Engineers) and the CCIF (International Telephonic Consulative Committee) tests.
The SMPTE test appears to have been introduced in the United States by Frayne and Scoville (see Frayne, J. G. and Scoville R. R., "Analysis and Measurement of Distortion in Variable-Density Recording", J.S.M.P.T.E., 32 (June, 1939) pp. 648-673) who considered the evaluation of distortion in variable density sound-on-film recording. The test signal utilized in the SMPTE test is a low frequency (between 40 Hz and 100 Hz) and a high frequency (between 1000 Hz and 12,000 Hz) tone, summed together in a 4 to 1 amplitude ratio. Other amplitude ratios are occasionally used.
The CCIF test which was introduced in 1933 by von Braunmuhl and Weber (see von Braunmuhl, H. J. and Weber, W., "Nichtlineare Verzerrungen von Mikrophonen", Elektrotechnische Zeitschrift, Vol. 54 (Nov., 1933)) differs from the SMPTE test in that a pair of equal-amplitude signals closely spaced in frequency are applied to the device under test. The nonlinearity in the device under test causes intermodulation products between the two signals which are subsequently measured. For the typical case of input signals at 14 kHz and 15 kHz, the intermodulation components are at 1 khz, 2 kHz, 3 khz, etc. and 13 kHz, 16 kHz, 12 kHz, etc. Even order or asymmetrical distortions produce the low frequency components while odd order or symmetrical distortions produce the frequency components near the input signals. Typically only the even order frequency components are measured.
Instruments have been designed which perform both the SMPTE and CCIF intermodulation tests. These analyzers, however, require an operator to manually switch between the two tests.